The present invention relates to superconductors, particularly to the current carrying capabilities of superconductors. It is a proven fact that the critical currents of many superconductors, here polycrystalline superconductors, are limited by grain boundaries formed in them. The invention overcomes this limitation of present superconductors by using alterations for improving the current transport properties of the superconductors"" grain boundaries. This is done in principle by doping these grain boundaries to dopant concentrations different from the concentrations of the grains, thus aiming at diminishing detrimental effects caused by space charge layers formed at the grain boundaries.
Based on the new class of superconductors, henceforth referred to as high-Tc superconductors, which were discovered by Bednorz and Mxc3xcller and disclosed in their article xe2x80x9cPossible High-Tc Superconductivity in the Baxe2x80x94Laxe2x80x94Cuxe2x80x94O Systemxe2x80x9d, Zeitschrift fxc3xcr Physik B, Condensed Matter, Vol. B64, 1986, pp.189-193, a variety of superconducting wires, cables and tapes have been developed for the transport of electrical current. A key parameter defining the performance and thus the economic benefit of these conductors is given by their so-called critical current density, which is the maximum density of the current these conductors can carry as so-called supercurrents in the superconducting state. The critical current density is a specific property for a given superconductor, and, for the practical use of a superconductor, one aims at maximizing this critical current density. Chaudhari et al. have taught in their article xe2x80x9cDirect Measurement of the Superconducting Properties of Single Grain Boundaries in YBa2Cu3O7xe2x88x92xcex4xe2x80x9d, Physical Review Letters, Vol. 60, 1988, pp.1653-1655, that the limiting factor for the critical current density of polycrystalline high-Tc superconductors is the electronic behavior of the boundaries formed by the crystalline grains of these materials. It was shown that the critical current densities of these grain boundaries are smaller by one to two orders of magnitude than the critical current densities of the grains abutting the grain boundaries.
Further, Dimos et al. have taught in their publication xe2x80x9cSuperconducting Transport Properties of Grain Boundaries in YBa2Cu3O7xe2x88x92xcex4 Bicrystalsxe2x80x9d, Physical Review B, Vol. 41, 1990, pp. 4038-4049, that superconducting grains with a small misorientation (below typically 8xc2x0 to 10xc2x0) behave as strongly coupled superconductors whereas larger misorientations (also called large-angle grain boundaries) are weakly coupled, showing Josephson junction-like properties. The teaching by Dimos et al. is the finding that the detrimental effect of the grain boundaries can be reduced by aligning the superconducting grains with respect to their crystalline main axes.
Following this proposal, wires and tapes of high-Tc superconductors have been fabricated, the critical currents of which are enhanced by aligning the superconducting grains by a variety of means, such as rolling processes or ion beam assisted techniques. Although these technologies have lead to the fabrication of high-Tc superconductors with current densities of the order of 100 000 A/cm2 at temperatures of 4.2 K, it remains desirable to fabricate high-Tc superconductors with still higher critical current densities or with processes which are less costly than the known ones.
Schmehl et al. in xe2x80x9cDoping Induced Enhancement of the Critical Currents of Grain Boundaries in YBa2Cu3O7xe2x88x92xcex4xe2x80x9d, Europhysics Letters, Vol.47, 1999, pp. 110-115; Mannhart et al. in European Patent Application EP 1 006 594 A1, xe2x80x9cSuperconductor with Enhanced Current Density and Method for Making such a Superconductorxe2x80x9d, and G. A. Daniels et al. in xe2x80x9cImproved Strong Magnetic Field Performance of Low Angle Grain Boundaries of Calcium and Oxygen Overdoped YBa2Cu3Oxxe2x80x9d Appl. Phys. Lett., Vol.77, 2000, pp. 3251-3253, have shown that the critical current densities of the grain boundaries can be enhanced by doping of the superconductor to dopant concentrations different from concentrations that would provide the optimum superconducting properties of the grains of the superconductors. By this, large grain boundary critical current densities are achieved. Because the critical temperature of the grains is reduced by this doping (see FIG. 1), the enhancement of the grain boundary critical current density is not obtained at 77 K (see FIG. 2) but only in the temperature range below. The temperature of liquid nitrogen, 77 K, is a preferable temperature for the operation of superconducting cables. As shown in xe2x80x9cDoping Induced Enhancement of the Critical Currents of Grain Boundaries in High-Tc Superconductorsxe2x80x9d by Mannhart et al, Physica C, Vol. 341-348, 2000, pp. 1393-1396, doping of YBa2Cu3O7xe2x88x92xcex4 with Co achieves the opposite effect and enhances the sample""s normal state resistance.
As described in the above-cited European Patent Application EP 1 006 594 A1, xe2x80x9cSuperconductor with Enhanced Current Density and Method for Making such a Superconductorxe2x80x9d, it was recognized that a preferential doping of the grain boundaries is advantageous in many cases.
The present invention now aims to achieve large critical current densities in a wide temperature regime, in particular in the range of 77 K and above, by avoiding the degradation of the grains due to their non-optimal doping. This different and novel approach according to the invention isxe2x80x94in briefxe2x80x94the teaching that by use of dopant multilayers, the critical current densities of the grain boundaries in a superconductor, and therefore also of the polycrystalline conductors, can be enhanced by doping the grain boundaries to dopant concentrations different from the dopant concentrations of the superconducting grains.
Ivanov et al. report in xe2x80x9cProperties of Locally Doped Bicrystal Grain Boundary Junctionsxe2x80x9d, Physica B, Vol. 194-196, 1994, pp. 2187-2188, experiments in which they investigated the effects of Fe and Pt embedded into grain boundaries in YBa2Cu3O7xe2x88x92xcex4 films on the transport properties of these grain boundaries. In this work, the grain boundaries were also doped to concentrations different from the dopant concentrations of the grains. This was achieved by using specially prepared bicrystalline substrates. Prior to the growth of the high-Tc superconductors, Fe and Pt dopants were embedded into the grain boundary of the substrate, which was a ZrO2 bicrystal. During the growth of the superconducting film, the substrate acted as a source of the dopants. In these experiments, a strong reduction of the critical current density was observed, i.e. just the opposite of the aim of the present invention. The critical current reduction in the Ivanov et al. experiment presumably took place due to the unsuited doping configuration as well as due to an unsuited selection of the dopants.
A general objective of the present invention is to provide a solution for an increased use of high-Tc and other superconducting materials by improving the current transport mechanisms within such materials, in particular for increasing the upper limit of the achievable supercurrent density within such materials.
A specific objective, as mentioned above, is to provide an approach by which the current carrying capabilities of superconductors, e.g. superconducting wires or tapes, can be significantly improved in a wide temperature range.
A further objective of the invention is to provide a technique for a simpler manufacturing process, resulting in cheaper mass production of polycrystalline superconductors with large critical current densities than with presently used techniques. At present, the fabrication of such superconductors requires cumbersome and costly processes to optimize the grain boundary geometries, such as improving their alignment or enhancing the effective grain boundary area.
A still further objective is to provide an approach for reducing the sensitivity of the critical current densities of polycrystalline superconductors to magnetic fields, which often exist in the environment in which the superconductor is operated or which are induced by the supercurrents themselves. Generally, the critical current densities of superconductors are easily reduced or suppressed by magnetic fields, which poses a severe problem for all applications in which the superconductors must be operated in magnetic fields, such as superconducting magnets or superconducting cables.
The novel solution taught by the invention is based on a better understanding of the transport mechanism within polycrystalline superconductors and consists essentially in chemically doping the grain boundaries of the superconductors to a dopant concentration different from the dopant concentrations of the grains.
This specific doping is achieved by using diffusion, in particular grain boundary diffusion, in e.g. multilayer structures as illustrated in detail below. This is done to modify space charge layers formed at the grain boundaries, e.g. by changing the density of the superconductor""s charge carriers or by modifying the built-in potential of the grain boundaries in an uncommon way. By this, the current transport properties of the grain boundaries are optimized without deteriorating the grains themselves as taught by Hammerl et al., Nature Vol. 407, 2000, pp.162-164.
The hereby achieved concentrations of dopant ions at the grain boundaries differ substantially from the concentrations levels typically used to fabricate polycrystalline conductors. The reason is that in the known approaches no care is usually taken to dope the grain boundaries to optimal dopant concentrations differing from those of the grains. The novel approach according to the invention changes this picture and the understanding of the current transport mechanism in superconductors, particularly high-Tc materials, especially with regard to the maximum (or critical) current density in such materials.
Another advantage of the approach according to the invention is that it simplifies the manufacturing process of superconductors for technical applications by allowing cheaper mass production of polycrystalline high-Tc and other superconductors with large critical current densities. The reason is that the novel approach reduces the complexity of steps and processes which are used today for optimizing the geometries of the grain boundaries, as mentioned above.
As grain boundaries embedded in superconductors are also frequently used in electronic device applications, e.g. in superconducting quantum interference devices, the possibility to alter the grain boundaries specifically presents a novel opportunity to tune the grain boundaries used in such devices to optimal properties.
A still further advantage is the fact that the critical current densities of superconductors made according to the invention show a reduced sensitivity to magnetic fields which quite often exist in the vicinity of the superconductor, either from other sources or induced by the supercurrents themselves. Thus, superconductors according to the invention can be securely and efficiently operated in environments where strong magnetic fields are abundant, such as superconducting magnets or superconducting cables.
In the following, an implementation of the invention shall be described supported by the appended drawings.